Ultrathin magnesium nanoblades

ABSTRACT

A nanostructure includes a plurality of metal nanoblades positioned with one edge on a substrate. Each of the plurality of metal nanoblades has a large surface area to mass ratio and a width smaller than a length. A method of storing hydrogen includes coating a plurality of magnesium nanoblades with a hydrogen storage catalyst and storing hydrogen by chemically forming magnesium hydride with the plurality of magnesium nanoblades.

This application is a Divisional of U.S. application Ser. No.12/594,047, filed Apr. 19, 2010, which claims benefit of priority toU.S. Application Ser. No. 60/921,329, filed on Apr. 2, 2007 and areincorporated herein by reference in their entirety.

This invention was made with Government support under contract number0506738 awarded by the National Science Foundation. The Government hascertain rights in the invention.

BACKGROUND

The present invention is directed to nanostructures in general and tometal nanoblades in particular. Oblique angle deposition has beendemonstrated as an effective technique to produce three-dimensionalnanostructures, such as nanosprings and nanorods; see for example Robbieet al., J. Vac. Sci. Technol. A, 15, 1460 (1997); Zhao et al., SPIEProceedings 5219, 59 (2003). Because of the physical shadowing effect,the oblique incident vapor is preferentially deposited onto the highestsurface features.

SUMMARY

A nanostructure comprises a plurality of metal nanoblades positionedwith one edge on a substrate. Each of the plurality of metal nanobladeshas a large surface area to mass ratio and a width smaller than alength.

In an alternative embodiment a method of storing hydrogen comprises (i)coating a plurality of magnesium nanoblades with a hydrogen storagecatalyst; and (ii) storing hydrogen by chemically forming magnesiumhydride with the plurality of magnesium nanoblades.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows SEM images of (a) a planar view of Mg nanoblades (b) a sideview of Mg nanoblades.

FIG. 2 shows a SEM top view image of one of the Mg nanoblades grown byoblique angle vapor deposition at a 75° incident angle with respect tosubstrate normal.

FIG. 3 provides schematics of hydrogenation: (a) Pd on a continuous Mgfilm, (b) Pd on columnar Mg film/Pd. The surface area to mass ratio for(a) and (b) is in the order of 10⁻² m²/g. (c) Atomic layer deposition ofPd on Mg nanoblades with surface area to mass ratio in the order of 60m²/g.

FIG. 4 shows top view SEM images of the Mg films deposited at vaporincident angles α of (a) 0°, (b) 30°, (c) 59°, and (d) 75°. Alpha (α) isthe vapor incident angle measured from the substrate normal. The insetin each figure shows a higher magnification view. The crystal axes,obtained from texture analysis, are indicated as white arrows in (d) andits inset. The scale bar in the insets is 200 nm except that in (d) is100 nm.

FIG. 5 presents (a) cross sectional SEM image of the Mg nanobladesdeposited at a vapor incident angle α of 75°. The black arrow indicatesthe tilted nanoblades at the beginning of the growth. α and β are thevapor incident angle and the nanoblade tilting angle measured from thesubstrate normal, respectively. (b) A plot of the vertical heights ofthe nanoblade films as a function of the vapor incident angle.

FIG. 6 shows (a) TEM bright field image of the Mg nanoblades depositedat a vapor incident angle of 75°. The black arrow indicates theparticular nanoblade that was selected for a detailed analysis. Theinset of (a) shows one of the selective area electron diffraction (SAED)patterns of the nanoblade. In addition to the diffraction spots fromcrystalline Mg there is ring structure from MgO. (b) The lattice fringeimage near the edge of the nanoblade. The interplanar distances for the(1 210) plane and (10 10) plan are measured to be 1.67 Å and 2.91 Å,respectively. About 2 to 4 nm thick oxidation region (MgO) adjacent tocrystalline Mg was seen after the sample was exposed in the air.

FIG. 7 presents in situ RHEED images of the Mg nanoblade films depositedat vapor incident angles of (a) 0°, (b) 30°, (c) 59°, (d) 75°. Thesimulated diffraction pattern for a (10 10) [0001] biaxial (II-O)texture was superimposed on the top of the measured diffraction patternin (d). α and β′ are the vapor incident and texture tilting anglesmeasured from the substrate normal, respectively.

FIG. 8 shows (a) top view SEM image of a thin Mg nanoblade filmdeposited at a vapor incident angle of 75° with a height of about 630nm. Black arrows indicate surface steps formed during the growth. (b) Aschematic of the growth modality for crystal growth with high surfacediffusion. Diffusions along the [0 110] and [21 30] directions areindicated by the movements of the atoms. The growth rate V along the [2130] direction can be divided into the vertical and parallel components,V_(⊥) and V_(∥), respectively. α, β, and β′ are the vapor incidentangle, nanoblade tilting angle, and texture tilting angle measured fromthe substrate normal, respectively.

FIG. 9 shows (a) top view SEM image of nanoblades with about 49.7minutes of deposition time (about 2.1 μm thick). The geometry of theRHEED measurement when the substrate at the azimuthal angle φ=0° (or180°) is indicated. The incident electron beam direction and incidentflux direction are 90° with respect to each other. φ is the azimuthalangle around the substrate normal. The electron beam is parallel to thewider width direction of nanoblades. (b) Side view SEM image ofnanoblades viewed from the direction perpendicular to the incident fluxdirection. β is the nanoblade tilting angle measured from the substratenormal. The thicknesses corresponding to various deposition times arelabeled by horizontal arrows. (c) A side view of the SEM image viewedfrom the direction parallel to the incident flux direction that is outof the page. The large surfaces of the nanoblade, facing away or towardsthe flux, are (0001) planes. The upward growth direction of a nanobladeis approximately along the [21 30] axis.

FIG. 10 shows RHEED patterns of Mg nanoblades when the electron beam isparallel to the wider width direction of nanoblades [sample position atφ=0° (or)180°] and perpendicular to the wider width direction ofnanoblades [sample position at φ=90° (or 270°)]. Phi (φ) is theazimuthal angle around the substrate normal. Portions (a) and (b) arefor deposition time of 0.5 minutes (about 22 nm thick), portions (c) and(d) are for deposition time of 13.7 minutes (about 589 nm thick), andportions (e) and (f) are for deposition time of 34.7 minutes (about 1.49μm thick). The normal of (10 10) planes and the [0001] axis areindicated by the white long dashed lines with arrows in (e). The shortdotted line with an arrow in (e) represents the normal of (10 11)planes.

FIG. 11 illustrates (a) a 3D construction of the reciprocal space fromthe RHEED patterns of the Mg nanoblade film deposited for 49.7 minutes(about 2.1 μm thick). Point A on the S_(z) axis is the cross pointbetween the (10 10) arcs. The polar angle θ is measured from thesubstrate normal S_(z) axis. φ is the azimuthal angle around thesubstrate normal. (b) The plot of the intensity of point A vs theazimuthal angle φ.

FIG. 12 shows the (10 10) RHEED pole figures of the Mg nanoblade filmwith a deposition time about 49.7 minutes. (a) Before a normalizationand (b) after a normalization; (c) and (d) show the corresponding (1011) RHEED pole figures before and after a normalization. The dashed linerepresents the azimuthal positions, where φ=0° (or 180°). The φ=0° meansa direction towards the incident vapor flux and the φ=180° means adirection away from the incident vapor flux.

FIG. 13 shows normalized (10 11) RHEED pole figures at deposition timesof (a) 0.5 minutes (about 22 nm thick), (b) 8.5 minutes (about 365 nmthick), (c) 24.5 minutes (about 1.05 μm thick), and 34.7 minutes (about1.49 μm thick). The positions of poles in the figures move towards theincident vapor flux as the film grows. The intensity of the azimuthalplot is around the white dashed circle which goes through (1 101), (1011), and (01 1 1) poles, as shown in (d). The center of circle is thegeometrical position of the [0001] axis.

FIG. 14 shows (a) the (10 11) polar intensity profiles at φ=0° (or 180°)of Mg nanoblades deposited for 8.5 minutes (about 365 nm thick), 13.7minutes (about 589 nm thick), 24.5 minutes (about 1.05 μm thick), 34.7minutes (about 1.49 μm thick), and 49.7 minutes (about 2.1 μm thick).The φ=0° means a direction towards the incident vapor flux and theφ=180° means a direction away from the incident vapor flux. The peakposition with the maximum intensity moves towards the incident vaporflux as the film grows thicker. This trend is indicated by a dashedarrow, which corresponds to the movement of the (10 11) pole positions.The inset of (a) is the plot of the texture axis tilting angle β′₍₁₀ ₁₁₎vs the deposition time t. (b) The intensity vs the azimuthal anglearound the [0001] axis (φ_([0001])) at different deposition times. Flatintensities on both sides of the curves represent the regions beyond theshadowing edges in RHEED patterns, which have zero intensity. The spikesat the center peaks labeled by arrows are due to the particles presentedon the surface. The base lines of intensity profiles in (a) and (b) havebeen shifted for clarity.

FIG. 15 illustrates schematics of views along the [0001] axis of thecrystals with the azimuthal angle orientation when (a) the [10 10] axisis along the vertical growth direction of a crystal and (b) the [21 30]axis is along the upward growth direction of a crystal. The two shadedregions above the dashed horizontal line are used as examples tocalculate the flux capture cross sections in the two different azimuthalangle alignments. The side length of a crystal is l. In the final film,the [0001] axis is tilted away from the substrate normal under obliqueangle deposition.

DETAILED DESCRIPTION

All references cited herein are hereby incorporated by reference intheir entirety.

Introduction

Oblique angle deposition (“OAD”) has been demonstrated to be aneffective technique to produce three-dimensional nanostructures, such asnanosprings and nanorods; see for example Robbie et al., J. Vac. Sci.Technol. A, 15, 1460 (1997); Zhao et al., SPIE Proceeding, 5219, 59(2003). In this technique, the vapor flux is incident at an obliqueangle α with respect to the substrate normal while the substrate isstationary or rotating around the surface normal. As a result of thephysical shadowing effect, the oblique incident vapor is preferentiallydeposited onto the highest surface features. This preferential growthdynamic gives rise to the formation of well-separated nanostructures.The dimension of these nanostructures generally ranges from tens tohundreds of nanometers, depending on the deposition material, depositionrate and the substrate temperature. For materials with high diffusivity,the initial nucleation density is low and this can lead to more isolatedstructures. It is generally believed that in this case the dimensions ofthe deposited structures are larger. In addition to the shadowing effectand diffusion, the intrinsic microstructure of the deposited materialalso affects the formation of the nanostructures.

In one embodiment of the present invention, the growth of magnesium (Mg)nanoblades by the oblique angle deposition with no substrate rotationwas observed. These nanoblades stand nearly vertically, thus deviatingfrom the well-known tangent and cosine rules for columnar structuresgrown by oblique angle deposition, as shown in FIG. 1; see for exampleDirks et al., Thin Solid Films 47, 219 (1997); Tait et al., Thin SolidFilms, 226, 196 (1993). FIG. 2 presents a SEM top view image of one ofthe Mg nanoblades grown by OAD at an 75° incident angle. The width ofthe Mg nanoblades along the incident vapor direction can be betweenabout 15 and about 80 nm; for example between about 15 to about 30 nm,while the dimension perpendicular to the incident vapor direction, orlength, can be as much as a few hundred nm, such as about 200 nm toabout 2000 nm. The thickness, or vertical height, is a function of thematerial, and can be between about 10 μm to about 500 mm. The width is afunction of the angle of vapor flux during deposition, and can be aboutbetween 15 nm to about 60 nm. The formation of these extremelyanisotropic structures with nanometer scale features shows that thegrowth of highly diffusive Mg deviates significantly from pastexperimental results and theoretical predictions. In addition to theanisotropic blade morphology, a biaxial (II-O) texture can be observedusing in situ reflection high energy electron diffraction (RHEED).Exemplary illustrations of such texture are provided in for exampleBauer, Fiber Texture, The Ninth Nat. Vacuum Symp. Am. Vac. Soc., editedby H. George and Bancroft, Macmillan, N.Y. (1963); Brewer et al., J.Appl. Phys. 93, 205 (2003). The surface area to mass ratio can reachbetween about 40 m²/g to about 60 m²/g, such as 52 m²/g, using anaverage width of about 15 to about 30 nm, such as about 22 nm. Thisvalue is about two orders of magnitude higher than that of ball-milledMg powders that contain nano-crystallites; see for example Zaluska etal., Appl. Phys. A 72, 157 (2001). The nanoblades have potentialapplications in many areas due to this very high ratio. For example, thenanoblades could be used as a metal hydride for hydrogen storage or as aphotocathode material, as illustrated in FIG. 3.

Unlike three-dimensional springs and rods, nanoblades are extremelythin, with very large surface areas. The large surface area featuremakes nanoblades particularly appealing for energy storage applications,particularly hydrogen storage. In order to store hydrogen, a largesurface area is desired to provide room for the material to expand asmore hydrogen atoms are stored. The vast surface area of each nanoblade,coupled with the large spaces between each blade, can make nanobladesdesirable for this application.

While OAD is the preferred deposition method, other suitable depositionmethods may also be used. Although Mg is preferred, nanoblades of othermetals or alloys can also be formed.

NON-LIMITING EXAMPLES Magnesium Crystalline Nanoblades Formed by ObliqueAngle Vapor Deposition

1.1 Experimental Details

An ultra high vacuum (UHV) thermal evaporation system was used todeposit Mg nanoblades. The Mg pellets (purity 99.95%) were placed in analuminum oxide crucible and heated resistively to the desiredtemperature of about 653 K for evaporation. To study the effect of thevapor incidence angle, the deposition was performed simultaneously onfour substrates mounted on a multi-angle sample holder with α=0°, 30°,59°, and 75°. The angle α is between the incident flux and the substratesurface normal. The substrates were p-type Si(100) with a thin layer ofnative oxide residing at the surface. The distance between evaporationsource and the substrate holder was approximately 10 cm. The basepressure of the vacuum chamber was about 3×10⁻¹⁰ Torr. The Mg source wasthoroughly degassed and evaporated for about 25 minutes prior to thedeposition. During the deposition the pressure rose to about 1.2×10⁻⁸Torr. The evaporation rate was maintained between 0.9 and 1.1 nm/s asindicated by a quartz crystal monitor (QCM). A K type thermocouple wasattached to the substrate holder and showed a temperature increased from303 K to 310 K during the deposition. The total deposition time was 90minutes. The crystalline structure on the surface of the deposited filmswas characterized in situ by RHEED. The microstructure of the Mg filmswas studied ex situ by field emission scanning electron microscopy (SEM)and transmission electron microscopy (TEM).

1.2 Results and Data Analysis

SEM and TEM Images of the Nanoblades

FIGS. 4( a) to (d) and the insets provide SEM top view images of thedeposited Mg films at different vapor incident angles. At normal vaporincidence, the film was continuous and comprised polygon shapedcrystals. The crystal sizes were on the order of a few micrometers. Themeasured angle between the edges of a crystal from the high-resolutionSEM image shown in the top right inset of FIG. 4( a) was about 125°.This suggests that these large crystals were the result of coalescenceof multiple HCP (0001) oriented crystals. Taking the ratio of thesubstrate temperature to melting temperature of Mg yielded a homologoustemperature of about 0.34 (T_(h)=T_(s)/T_(m)=310 K/923 K). This value ofabout 0.34 indicates that the growth of Mg should be in Zone II regime(0.3<T_(h)<0.5), according to the structural zone model of depositedfilms; see for example Thornton, J. Vac. Sci. Technol. 12, 830 (1975);Barna et al., Growth Mechanisms of Polycrystalline Thin Films, Scienceand Technology of Thin Films, edited by Matacotta and Ottaviani, WorldScientific (1995). The growth in Zone II regime implies a high atomicmobility, which is consistent with the large crystals observed in FIG.4( a). At a vapor incident angle of 30°, the film started to form astripe shaped structure perpendicular to the vapor incident direction(see FIG. 4( b)). The width of the stripe along the vapor incidentdirection was about 250 to about 300 nm. As the deposition angleincreased, the width of the stripes along a vapor incident directiondramatically decreased, forming thin nanoblades. In one embodiment, atthe vapor incident angle of 59°, the width of the nanoblades along thevapor incident direction varied from about 30 to about 150 nm. At avapor incident angle of 75°, the nanoblades became ultrathin with awidth of about 15 to about 30 nm. The dimension perpendicular to thevapor incident direction, or length, was on the order of a few hundrednm. A typical nanoblade is shown in the top right inset of FIG. 4( d),wherein the surface steps along the width of the nanoblades can be seen.

FIG. 5( a) shows the cross-sectional SEM image of the Mg nanoblade filmdeposited at 75°. The nanoblades were almost vertical and ran throughthe entire thickness of the film except near the substrate. In the earlystage of growth (less than about 1 μm), some nanoblades tilted away fromthe flux. The tilting angle of nanoblades was defined as β, which wasmeasured from the substrate normal as indicated in FIG. 5( a). Thistilting and vertical growth of the nanoblades deviated from the tangentand cosine rules that related β to the incident flux angle α duringoblique angle deposition. An illustration of the height of the film withrespect to the vapor incident angle is provided in FIG. 5( b). Theheight along the substrate normal varied from a few μm to about 21 μm asthe incident flux angle increased from 0° to 75°. The films deposited atthe two largest incident vapor angles were much taller than the filmdeposited at normal vapor incidence, even though they received lessflux. It has been observed that films such as Co deposited at normalvapor incidence are thicker than films deposited at oblique angles; seefor example Tang et al., J. Appl. Phys. 93, 4194 (2003). Thedisagreement came from the vertically oriented nanoblades versus theconventional slanted nanorods grown by oblique angle deposition. Theunexpected height of the nanoblades was accompanied by the extremelyporous nature of the film. The nanoblades can be separated from oneanother by about 0.5 to 3 μm, such as about 1 to about 2 μm, as shown inFIGS. 4( c) and (d).

The single crystal nature of a single Mg nanoblade was investigatedusing TEM. FIG. 6( a) shows the bright field TEM image of the Mgnanoblades that were deposited at an incident vapor angle of about 75°.The black arrow indicates the nanoblade selected for a detailedanalysis. One representative selective area electron diffraction (SAED)pattern along the (0001) zone axis is shown in the upper right inset ofFIG. 6( a). In addition to the spot pattern, two weak diffraction ringsfrom polycrystalline MgO are labeled by white arrows in the diffractionpattern. The oxidation of the sample occurred after exposure to air.FIG. 6( b) is a high resolution transmission electron microscopy (HRTEM)image taken near an edge of the nanoblades. The Mg lattice fringe can beseen in the image. The measured distances between lattice fringes in twoorthogonal directions were about 2.91 Å and about 1.67 Å. These twovalues were close to the (1010) and (1210) interplanar distances of theMg hexagonal close-packed (HCP) structure, which were 2.78 Å and 1.61 Å,respectively. Additionally, FIG. 6( b) shows that an oxidation layerwith a thickness of about 2 to about 4 nm is adjacent to the Mg crystal,and the interface between Mg crystal and oxide is diffusive.

RHEED Patterns and Texture Analyses

In addition to the ex situ characterization of the Mg films, an in situRHEED was used to study their crystalline orientations. As shown in theSEM top view images, the nanoblades were well aligned, with the widersides approximately perpendicular to the vapor flux direction and thethin sides parallel to the vapor flux direction. This suggests that theMg nanoblades had two preferred crystalline orientations, (i.e., abiaxial, or II-O texture). The biaxial texture has been often observedin the films grown by oblique angle deposition; see for example, Alouachet al., J. Vac. Sci. Technol. A 22, 1379 (2004); Tang et al., Phys. Rev.B. 72, 035430 (2005); Morrow et al., J. Vac. Sci. Tech. A 24, 235(2006). FIGS. 7( a) to (d) provide in situ RHEED images of the Mg filmsdeposited at different angles. The diffraction patterns of films grownat oblique angles were composed of distributed arcs that indicated awell-developed texture. In one embodiment, the diffraction pattern forα=75° was examined, as shown in FIG. 7( d). The ratios between thevarious radii of the diffraction arcs in FIG. 7( d) are1:1.13:1.46:1.72:1.88, indicating that the diffraction arcs were fromthe (10 10), (10 11), (10 12), (11 2 0), and (10 13) planes. The latticeconstants a and c measured from the diffraction pattern were 3.18±0.10 Åand 5.20±0.17 Å. Both of the measured values were close to those of theMg crystal, which were 3.21 and 5.21 Å, respectively. Through theanalyses of the angles between different diffraction arcs, the (10 1 0)[0001] biaxial texture was found to form in the Mg film. In this biaxialtexture, the preferred orientations were the surface normal of the (1010) plane and the [0001] axis. The simulated diffraction pattern forthis biaxial texture was superimposed on the top of the diffractionpattern in FIG. 7( d). The (10 10) texture axis, namely the [21 30]axis, is almost normal to the substrate, while the [0001] axis pointsalong the vapor flux direction with an angle β′=85°. Beta prime (β′) isthe tilting angle of the [0001] axis as measured from the substratenormal. This β′ angle (85°) suggests that the side faces of thenanoblades perpendicular to the flux direction are (0001) crystallineplanes. Orthogonal to the [0001] and [21 30] axes is the [01 10] axis.These three crystal axes can serve as a spatial reference. These axesare also indicated by the arrows shown in FIG. 4( d). The β′ decreasedto 79° and 45° as the incident flux angle α decreased from about 59° toabout 30°, respectively. At the normal vapor incidence, the diffractionpatterns in FIG. 7( a) showed a weak vertical (10 11) fiber texture andan absence of {0001} planes parallel to the substrate.

The RHEED analysis indicates that the side surface of the nanobladesthat faced the flux was an (0001) plane. This plane is the most compactcrystalline plane in the hexagonal close packed (HCP) structure, whichhas the lowest surface energy. Since the nanoblades stand vertically,the (0001) surface faces the vapor flux, and consequently receives themajority of the flux. The formation of these long and thin nanobladesimplies that the atoms deposited on the (0001) surface diffuse readilyto the edges of this face and are transported to the adjacent surfaces,which are parallel to the vapor flux; see for example Gilmer et al.,Thin Solid Films 365, 189 (1999); Liu et al., Appl. Phys. Lett. 80, 3295(2002). The adjacent surfaces are higher surface energy planes such as{10 10} planes, according to the equilibrium crystal structure; see forexample Bauer, Growth of Oriented Films on Amorphous Surfaces inSingle-Crystal Films, edited by Francombe and Sato, Macmillan, N.Y.(1964). Additionally, the SEM image in FIG. 4( d) shows the stepstructure on the crystal surface. These steps are more clearly seen inthe initial growth of the crystal. FIG. 8( a) shows a top view SEM imageof a thick film of about 630 nm, deposited at a vapor incident angle of75°. In this image, a series of steps along the [0 110] direction arevisible on the surface of the nanoblade, indicated by the black arrows.The step structure could indicate a high density of planer defects suchas stacking faults parallel to the (0001) plane; see for exampleSeryogin et al., Nanotechnology 16, 2342 (2005); Levin et al., Appl.Phys. Lett. 87, 103110 (2005).

Thickness of the Nanoblades Along the Incident Vapor Flux Direction

Based on the experimental observations, in FIG. 8( b) a schematic of theproposed growth model is provided. After atoms land on the (0001)surface, they diffuse isotropically. This leads to the growth of thecrystal along the [0 110] and [21 30] directions. The growth along the[01 10] direction results in the disproportional width of the nanobladein the direction perpendicular to the vapor flux. The growth along the[21 30] direction contributes to the vertical growth of the nanoblades.As the atoms are transported to the adjacent {10 10} surfaces thediffusion would be slower compared to that on the (0001) plane. Thislower diffusion will lead to a reduced thickness of the nanoblades.However, the difference in the diffusion on these planes is usuallysmall. Surface defects such as steps observed in the morphology of thenanoblades may be an important factor in accounting for the formation ofthe ultrathin nanoblades. Since the {10 10} faces have a higher surfaceenergy, the transported atoms will prefer to stay on these planes asshown in FIG. 8( b). They may not have sufficient thermal energy to moveover the nearby step onto the (0001) plane. The width of the stepplateau along the [0001] axis (i.e., the vapor flux direction) candetermine the width of the nanoblade in the flux direction. A systematicmolecular dynamics (MD) simulation can be helpful to understand thedetailed mechanism of the surface diffusion and the formation of thesurface steps.

Tilting Angles of the Texture Axis and Nanoblades

Without wishing to be bound by a particular theory, the relationshipbetween the tilting angle of the (0001) plane and the vapor incidentangle can be understood using the van der Drift theory for highlydiffusive surfaces; see for example van der Drift, Philips res. Rep. 22,267 (1967). Based on this theory, the nanoblades with the highestvertical growth rate can survive. For the nanoblade, the vertical growthrate is a result of the diffusion along the [21 3 0] direction. Thetotal material received by the crystal is ˜cos(α−β′), so that the growthrate V along the [21 30] direction is also ˜cos(α−β′). The verticalcomponent V_(⊥) of the growth is ˜cos(α−β′)sin β′. V_(⊥) is maximizedwhen β′=45°+½α. Using the values of α=30°, 59°, and 75°, β′ iscalculated to be 60°, 74.5° and 82.5°, respectively. It was observedthat at high vapor incident angles α, (i.e., 59°, and 75°), thepredicted β′ values 74.5° and 82.5° were close to the experimental β′values of 79° and 85°. However, for α=30° the experimental β′ value was45°, which was much smaller than the predicted value of 60°. This can bedue to the reduced influence of the shadowing at smaller α angles. Asthe shadowing effect was weakened the crystals with smaller tiltingangles were not suppressed. Since the nanoblades grow slowly along the[0001] axis, the growth along the [21 30] axis caused the blades to tiltaway from the flux at an angle β=90−β′, where β is measured from thesubstrate normal. The tall nanoblades that survived have large β values,indicating that those nanoblades stood nearly vertically. However, atthe early stages of growth, the nanoblades with a large tilting angle β(i.e., small β′) can also exist, as shown in the cross sectional SEMimage in FIG. 5( a). Markus Bauer et al. have observed that underoblique angle e-beam deposition, the MgO rods tilted slightly away fromthe vapor flux; they argued that this abnormal tilting angle was due toa directional diffusion originated from the momentum of the incidentvapor atoms; see for example Bauer et al., Mat. Res. Soc. Symp. 587,O2.2.1 (2000). In one embodiment, for the 75° incident, Mg flux wasclose to a normal incidence on the (0001) surface of the nanoblades. Inaddition, the kinetic energy of the Mg vapor atoms was very small atabout 0.056 eV, so that directional diffusion should be negligible; seefor example Sanders et al., Surface Science 254, 341 (1991); Sanders etal., J. Vac. Sci., Tech. A 10, 1986 (1992).

In some embodiments, palladium (Pd) can be coated on the nanoblades as ananocatalyst in situ by vapor evaporation or ex situ by atomic layerdeposition to lower the hydrogen adsorption/desorption temperatures. Anyother liquid or vapor phase deposition methods may also be used. Throughthis approach a new class of materials for reversible absorption anddesorption of hydrogen at moderate temperature and pressure withextremely high storage capacity can be developed. Other catalysissuitable for hydrogen storage, such as vanadium, platinum, niobium, orcobalt can also be used. Other applications, such as electron emitters,photoemission electron cathodes, can also be fabricated with this classof material.

In one embodiment, the Mg nanoblades, formed by OAD (see FIG. 2) had asurface area to mass ratio of about 60 m²/g. This estimated value camefrom these parameters: length of the nanoblade, about 20 μm, volume ofthe nanoblade, 630 nm×18 nm×20 μm=2.26×10⁻¹³ cm³, density: 1.738 g/cm³,and mass: 3.928×10⁻¹³ g. These numbers gave a surface area of about 630nm×20 μm×2=2.5×10⁻¹¹ m². Ratio of surface area/mass=63 m²/g. Note thatin some other embodiments, this ratio can be 45 m²/g or higher, and canrange from about 50 m²/g to about 70 m²/g. This surface area to massratio was approximately decided by the width (about 18 nm) of thenanoblade. This value was at least about three orders of magnitudehigher than that of a continuous Mg film made by normal vapor incidentdeposition or about two orders of magnitude higher than that ofball-milled Mg powders that contain nanosize grains. In addition, sincethe Mg nanoblades have width of less than 60 nm, such as about 20 nm,hydrogen dissociated by nanocatalyst Pd can diffuse through the wholethickness of the nanoblades to form Mg hydride, and the time required inhydrogenation and de-hydrogenation processes in these nanoblades can beshort (see FIG. 3). The Mg nanoblade theoretically can take up to 7.6wt. % hydrogen. The gaps between vertically standing, isolatednanoblades can accommodate between about 20% to about 40%, such as 30%,volume expansion between Mg and magnesium hydride during cycling. Thismay extend durability and cycle lifetime. Atomic Layer Deposition (ALD)is a desirable technique for a conformal uniform coating of a materialbecause it is a precisely controlled deposition technique for metals, asshown in FIG. 3. In one embodiment, ex situ ALD is used to coat the Mgnanoblades conformally with Pd. Pd can be deposited by this method onmetal and oxidized metal surfaces such as on air-exposed Ta and Siwithout the use of plasma.

2. RHEED Surface Pole Figure Study

2.1 Experimental Details

An ultrahigh vacuum (UHV) thermal evaporation system was used to deposita Mg film. The substrates were p-type Si(100) with a thin layer ofnative oxide residing at the surface. The vapor incident angle α withrespect to the substrate normal was about 75°. The distance between theevaporation source and the substrate holder was approximately 10 cm. Thesource was resistively heated and kept to a desired temperature of about600 K for evaporation. The base pressure of the vacuum chamber was about4×10⁻⁹ Torr. During deposition the pressure rose to about 2.0×10⁻⁸ Torr.The RHEED gun was operated at 8 kV and 0.2 mA emission current. A totalof 200 RHEED patterns covering the azimuthal angle of 360° with a stepof 1.8° was recorded for the pole figure measurements and took about 20minutes. Details of the setup and operation of RHEED can be found in forexample Tang et al., Appl. Phys. Lett. 89, 241903 (2006). The Mgdeposition was interrupted at 0.5, 4.5, 8.5, 13.7, 24.5, 34.7, and 49.7minutes for in situ RHEED pole figure measurements. The morphologies andstructures of the final Mg films were imaged ex situ by a field emissionSEM. The height of the Mg film obtained from the side view of SEM imageswas about 2.1 μm, rendering a growth rate of about 43 nm/minutes. Theheight refers to the vertical distance between the substrate and the Mgfilm surface. The thickness, or vertical height generally does not havea maximum value, and can range from for example about 500 nm to about 25μm. The width refers to the dimension along the vapor flux direction,and generally can range for example from about 15 nm to about 80 nm,such as about 15 nm to about 30 nm. The length refers to the dimensionperpendicular to both the thickness and the width, and generally canrange from about 200 nm to about 2000 nm.

2.2 Results and Data Analysis

SEM images

OAD of Mg film can be generally significantly different from othermaterials because of the fast diffusion of the Mg atoms. FIGS. 9( a)through (c) show SEM images of the final Mg film deposited for 49.7minutes at a vapor incident angle of 75°. The film is composed of manynanoblades (see FIG. 9( a)). The width of the typical Mg nanobladesalong the incident vapor direction ranged from about 15 to about 30 nm,while the length perpendicular to the incident vapor direction can be aswide as a few hundred nanometers. The side view image in FIG. 9( b)shows that the nanoblades tilted away from the incident flux. Thethicknesses corresponding to various deposition times are labeled byhorizontal arrows in FIG. 9( b). The competitive growth among thenanoblades occurred during deposition, in which nanoblades with lesstilting angle were favored. The top surfaces of less tilted nanobladeshad brighter contrast (see FIG. 9( a)), indicating that these nanobladesgrew taller and survived under the shadowing effect. FIG. 9( c) showsthe SEM cross sectional image viewing parallel to the incident fluxdirection that is out of the page. From this image it can be seen thatthe shape of a nanoblade is polygon. The very upper part of a nanobladeshows a combination of multiple partial hexagons and is highlighted bydashed lines. This suggests that the large surfaces of the nanoblade,facing away or towards the flux, were (0001) planes. Additionally, theupward growth directions of nanoblades were approximately aligned alongthe [21 30] direction. The surface normal of the (10 10) plane, alongthe [21 30] axis, was another preferred crystallographic orientation.The analysis of SEM images indicates the formation of a (10 10)[0001]biaxial texture, which is consistent with the pole figure analysis.

RHEED Patterns and Pole Figure Analysis

RHEED Patterns

In the RHEED pole figure measurement, the substrate was rotatedazimuthally with angle φ around the substrate normal while thedirections of the vapor flux and electron beam were fixed at 90° to eachother. The geometry of the RHEED measurement at an azimuthal angle φ of0° (or 180°) is shown in FIG. 9( a), where the azimuthal angle φ of 0°(or 180°) was defined as parallel to the incident flux direction, andthe direction of the incident electron beam is 90° with respect to theflux direction as a φ of 90° (or 270°). In this geometry the electronbeam direction was parallel to the wider width direction of nanoblades.As the substrate was rotated, the φ angle changed with respect to theincident electron beam direction, indicating that the wider widthdirection of nanoblades was rotated with respect to the electron beamdirection. FIG. 10 shows the RHEED images at selected deposition timesof 0.5 minutes (about 22 nm thick), 13.7 minutes (about 589 nm thick),and 49.7 minutes (about 2.1 μm thick). FIGS. 10( a), 10(c), and 10(e)are patterns when the wider width direction of nanoblades is parallel tothe electron beam [substrate positioned at φ=0° (or 180°)]. When thesubstrate was rotated to φ=90° (or 270°), the wider width direction ofnanoblades was perpendicular to the electron beam direction. Thecorresponding RHEED images are shown in FIGS. 10( b), 10(d), and 10(f).FIGS. 10( a) and 10(b) show almost continuous and uniform diffractionrings appearing at the very initial stage of growth less than 0.5minutes (about 22 nm thick), indicating a nearly random nucleation. Withmore deposition the diffraction rings became sharper and broke into moreparts as shown in FIGS. 10( c) and 10(e), indicating texture formation.At the later stage of growth, it can be seen that when the wider widthdirection of nanoblades was parallel to the electron beam, the patternsbecame asymmetric about the substrate normal (see FIGS. 10( c) and10(e)). This asymmetry resulted from the oblique angle incident vapor.The ratio between the various radii of the diffraction arcs in FIG. 10(e) is 1:1.14:1.45:1.72:1.87, similar to that shown in FIG. 7( d). Thisratio indicate that the diffraction arcs were from the (10 10), (10 11),(10 12), (11 2 0), and (10 13) planes. The lattice constants a and cmeasured from the diffraction pattern were 3.2±0.1 and 5.2±0.2 Å,respectively. These measured values were close to the lattice constantsof bulk Mg crystals, which were 3.21 and 5.21 Å, respectively. From theRHEED image analysis, it is seen that the (0002) diffraction arcs aremissing. This could be due to the electron refraction effect originatingfrom the inner potential of a crystal. The intensity of (0002)diffraction arcs mainly came from the electron beam parallel to the(0001) crystal surface. However, when the electron beam was nearlyparallel to the crystal surface, the refraction effect became thestrongest; an exemplary illustration is provided in for example Thomsonet al., Theory and Practice of electron Diffraction, Macmillan, N.Y.(1939). Almost no reflection is possible for which G² ₍₀₀₀₂₎/4k²<φP,where k is the wave vector of an electron beam, G₍₀₀₀₂₎ the reciprocalwave vector of the (0002) plane, φ the inner potential, and P is thepotential of an electron. Substitution of P=8000 V, k=45.82 Å⁻¹, andG₍₀₀₀₂₎=2.41 Å⁻¹ into the above inequality showed that the innerpotential of Mg was larger than 5.5 eV. Although a value of the innerpotential of Mg was not found in the existing literature, most of themetals have been shown to have a value of the inner potential largerthan 10 eV; see for example Vainshtein, Structure Analysis by ElectronDiffraction, Macmillian, N.Y. (1964).

Normalization of RHEED Pole Figure

The morphology of the Mg film is generally very anisotropic. Thisanisotropic morphology can severely distort the RHEED pole figure, whichis constructed from the polar intensity profiles at different azimuthalangles. A method of intensity normalization has been developed toaccount for this geometrical effect. For RHEED, a typical wave vector ofthe incident electron beam k is much larger than the interestedreciprocal lattice vector G; therefore the Ewald sphere could beapproximated as a plane. FIG. 11( a) shows a three-dimensional (3D)diagram of the constructed reciprocal space from the RHEED patterns ofthe Mg film deposited at 49.7 minutes (about 2.1 μm thick). Under theassumption that the Ewald sphere is approximated as a plane, the RHEEDpatterns at different azimuthal angles may have a common intersectionline along the substrate normal (i.e., S_(z) axis). Particularly, thepoint A on the S_(z) axis is the cross point between the (10 10) arcs.Although in theory the intensity of point A obtained from RHEED patternsat different azimuthal angles should have the same intensity, thisintensity varied significantly with respect to the azimuthal angles, asshown in FIG. 11( b). The valley of the intensity plot was around φ=0°(or 180°), while the peak intensity was around φ=90° (or 270°). Thisintensity modulation in azimuthal angles was likely due to theanisotropic morphology of the film. From the SEM image, the film is seento comprise well aligned nanoblades with the wider surface (face)perpendicular to the vapor flux. During the measurement, when anelectron beam is incident parallel to the wider width direction ofnanoblades, gaps between the nanoblade rows are exposed to the incidentelectrons that allow more electron channeling into the depth of thematerial. The channeled electrons can be captured in the bulk orcontribute to the background, resulting in a weak intensity at φ=0° (or180°). However, when an electron beam is perpendicular to the widersurface of nanoblades, φ=90° (or 270°), the electrons are perpendicularto the channeling gaps and the diffraction intensity becomes strongest.Both arguments are consistent with the observations from FIG. 11 (b).

For compensating the intensity modulation around azimuthal angles, shownin FIG. 11( b), the intensity of point A from different RHEED images wasnormalized to a constant value. FIGS. 12( a) and 12(b) show the (10 10)RHEED pole figures of the film deposited at 49.7 minutes before andafter normalization, respectively. The intensity around φ=0° (or 180°),indicated by the dashed line, was obtained from RHEED images when theelectron beam was incident parallel to the wider surface of thenanoblades. Before normalization, this intensity was significantly lowerthan the surroundings due to the intensity modulation described above.After normalization a center pole became visible along the dashed line.The pole structure suggests the formation of a biaxial texture. Throughthe analyses of the angles between different diffraction poles, a (1010)[0001] biaxial texture formation was observed in the Mg nanobladefilm; an exemplary analysis method is provided in for example Tang etal., Appl. Phys. Lett. 89, 241903 (2006). The calculated positions ofthe diffraction intensity for this biaxial texture were superimposed onthe top of FIG. 12( b) as solid squares.

A similar normalization process was followed by the construction ofother pole figures. FIGS. 12( c) and 12(d) show the (10 11) RHEED polefigures before and after the normalization, respectively. Afternormalization, the split center poles shown in FIG. 12( c) are mergedinto a single pole shown in FIG. 12( d). The positions of the individualpoles match closely with the theoretical positions of (10 10)[0001]biaxial texture. Since the diffraction intensity of the (10 11)diffraction ring is always strong during the texture evolution, theanalyses concentrate on the (10 11) pole figure.

Evolution of Normalized RHEED Pole Figures

FIGS. 13( a) through 13(d) show a series of normalized (10 11) RHEEDpole figures after 0.5, 8.5, 24.5, and 34.7 minutes of deposition. Inthe beginning of 0.5 minutes of deposition, the distribution ofintensity in the pole figure intensity was nearly even, indicating arandom initial nucleation on the amorphous substrate. With moredeposition at 8.5 minutes, an intense band was shown at the left side ofthe pole figure in FIG. 13( b). Clearly separated poles were revealed inthe longer deposition time of 24.5 minutes (about 1.05 μm thick). Theposition of the poles in the figures moved towards the flux as the filmgrew. This indicates that the texture axes have tilted more towards theflux. This change of the texture axis can be quantitativelycharacterized by the evolution of polar intensity profiles, measuredfrom RHEED images at φ=0° (or 180°). The plots are shown in FIG. 14( a).At the deposition time of 8.5 minutes (about 365 nm thick) the positionof the peak with the maximum intensity was about θ=43° on the side atφ=180°. As the film grew thicker, the peak position gradually moved tothe center. At the deposition time of 49.7 minutes, the texture axistilted to θ=1° on the side at φ=0°. The inset of FIG. 14( a) presentsthat the texture axis tilting angle β′₍₁₀ ₁₁₎ versus the deposition timet. The minus sign indicates that the texture axis tilts towards theincident vapor flux. It is seen from the figure that the texture axistilting angle changes most dramatically at the early stage of growth.

In addition to the movement of the pole position, the (1 101), (10 11),and (01 11) poles lie on a circular band can be observed (dashed curve)in FIG. 13( d). This circular band indicates that the variation of theazimuthal angle orientation was mainly around the [0001] axis. This is anatural consequence if considering that the (0001) plane is the majorflux receiving surface. FIG. 14( b) shows the intensity versus theazimuthal angle around the [0001] axis or φ_([0001]) at differentdeposition times. The φ_([0001]) was around the dashed circle which wentthrough (1 101), (10 11), and (01 11) poles; an example is provided inFIG. 13( d). The center of the circle will be the geometrical positionof the [0001] axis. This azimuthal plot is different from one that isaround the center of the pole figure. Three peaks in the azimuthal plotscorrespond to these three poles. Due to the scattering from theparticles present on the surface, some RHEED images within a narrowazimuthal angular region were severely distorted. This resulted in thespike observed in the center peak, which was most obvious in the curvesof the early stage of growth labeled with arrows. However, the sidepeaks clearly show the shrinkage of the peak width from about 44° toabout 27° during the film growth, indicating that the azimuthal angleorientation around the [0001] axis became more confined.

Texture Axis Tilting Angle

The pole figure analysis shows that the texture axes tilted more towardsthe incident vapor flux as the film grew. A similar change in thetexture axis orientation was also observed in a CdS film by OAD; see forexample Laermans et al., Thin Solid Films 15, 317 (1973); Hussain, ThinSolid Films 22 S5 (1974). The hexagonal shape of the nanobladesindicates that an individual nanoblade has a single crystal structure.This single crystal nature was also confirmed by transmission electronmicroscopy (TEM) analysis; see for example Tang et al., Journal ofNanoscience and Nanotechnology 7, 3239 (2007). Therefore, the variationof the texture axis tilting angle can be correlated to the change of thenanoblade tilting angle. This is consistent with the observations in themorphology of the final deposited film. From the SEM side view in FIG.9( b), the competitive growth among nanoblades is revealed. At the earlystage of deposition, nanoblades obviously tilted away from thesubstrate. For the nanoblades having large tilt angles, the growthdirections would significantly deviate from the substrate normal, andtherefore their vertical growth rates decrease and eventually stopgrowing. As a consequence, the texture axes would begin to tilt moretowards the incident vapor flux during the growth. The tilting angle ofthe nanoblade β can be assumed equal to the tilting angle of the (10 10)texture axis, β′₍₁₀ ₁₀₎. The β′₍₁₀ ₁₀₎ can be related to β′₍₁₀ ₁₁₎through β′₍₁₀ ₁₀₎=β′₍₁₀ ₁₁₎+28.7°. The 28.7° is the angle between thecrystalline (10 10) and (10 11) planes. The final value of β′₍₁₀ ₁₁₎obtained from FIG. 13( a) is −1. Therefore the final value of β′₍₁₀₁₀₎˜−1°+28.7°=27.7°. The final tilting angle of nanoblades measured fromFIG. 9( b) is ˜(22±6)°, which is comparable with the β′₍₁₀ ₁₀₎ of 27.7°.In addition, the [0001] texture axis is tilted from surface normal by˜−(1°±61.3°)=−62.3°, where the value of 61.3° is the angle between thecrystalline (10 11) and (0001) planes. Here the minus sign means thatthe texture axis tilts towards the incident vapor flux. The normal of(10 10) plane and the [0001] axis are labeled as long dashed lines witharrow heads in the RHEED pattern of FIG. 10( e). The short dotted linewith an arrow head in (e) represents the normal of (10 11) plane.

Alignment of Azimuthal Angle Orientation

A number of arguments, including crystal geometries (see for exampleTang et al., Phys. Rev. B 72, 035430 (2005); van der Drift, Philips res.Rep. 22, 267 (1974)), flux capture cross section (received flux) (seefor example Chudzil et al., IEEE Trans. Appl. Supercond. 11, 3469(2001)), and asymmetric surface diffusion (see for example Karpenko etal., J. Appl. Phys. 82, 1397 (1997)), have been presented in theliterature to explain the azimuthal angle selection in films depositedby OAD, but a general theory to accurately predict the orientationselection has not emerged. In one embodiment, there are two extremecases in the alignment of azimuthal angle orientation around the [0001]axis, as shown in FIGS. 15( a) and 15(b). The nanoblade was simplifiedas a hexagonal shape crystal and view along the [0001] axis. One waswith the [10 10] axis along the vertical growth direction of the crystal[FIG. 15( a)]; the other was the [21 3 0] axis along the vertical growthdirection of the crystal [FIG. 15 (b)], which was observed in oneembodiment. The [0001] axis was tilted away from the substrate normalunder OAD. The first case has a sharp tip pointing up. By a geometricalconsideration, the growth along a sharp tip (i.e., the [10 10] axis) canhave a faster growth rate than the direction perpendicular to a crystaledge (i.e., [21 3 0] axis); see for example Tang et al., Phys Rev. B 72,035430 (2006), which was contrary to the observation in one embodiment.The inconsistency can be resolved if the difference between flux capturecross sections of these two geometries was considered. Due to theshadowing effect, only the very upper part of crystals, for example, theshaded regions above the dashed horizontal lines, may be considered inthe calculation of the received flux. In this case the crystal in FIG.15( b) will have a much larger vapor flux capture cross section thanthat of the crystal shown in FIG. 15( a). Since the received flux wastransported to the side faces in the thin blade structure, thosecrystals with the [21 3 0] axis along the vertical growth direction canhave a higher vertical growth rate and survive. As an example, thecapture cross section when the cut off position of the vapor flux wascalculated and is labeled with the dashed horizontal lines in FIG. 15.The distance from the highest point of the crystal to the dashed line is½, where 1 is the crystal side length. The calculated flux capture crosssection areas (the shaded regions) are √{square root over (3)}/4l² and(2+/√{square root over (3)}/4)l² in FIGS. 15( a) and 15(b),respectively. The area in FIG. 15( b) is about 1.49 higher than the areain FIG. 15( a).

Therefore, metal nanoblades with a large surface area can be produced byoblique angle deposition. These vertically freestanding nanobladesexhibit a (10 10)[0001] biaxial texture. The morphology of thesenanoblades can be a function of the properties of the material and thedeposition conditions. These nanoblades can be further coated with acatalyst for applications such as hydrogen storage.

The foregoing description of the embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teaching or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and as a practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodification are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed:
 1. A nanostructure comprising a plurality of metalnanoblades comprising magnesium nanoblades each positioned with one edgeon a substrate.
 2. The nanostructure according to claim 1, wherein eachof the metal nanoblades of the plurality has a width which is smallerthan its length.
 3. The nanostructure according to claim 1, wherein eachof the metal nanoblades of the plurality is coated with a hydrogenstorage catalyst selected from palladium, cobalt, platinum, niobium orvanadium.
 4. The nanostructure according to claim 1, wherein a surfacearea to mass ratio of each of the metal nanoblades of the plurality isbetween about 50 and about 70 m²/g.
 5. The nanostructure according toclaim 1, wherein a surface area to mass ratio of each of the metalnanoblades of the plurality is larger than about 45 m²/g.
 6. Thenanostructure according to claim 1, wherein a width of each of the metalnanoblades of the plurality is between about 15 nm and about 30 nm. 7.The nanostructure according to claim 1, wherein a length of each of themetal nanoblades of the plurality is between about 200 nm and 2000 nm.8. The nanostructure according to claim 1, wherein each of the metalnanoblades of the plurality has a thickness of about from 500 nm toabout 25 μm.
 9. The nanostructure according to claim 1, wherein a lengthto width ratio of each of the metal nanoblades of the plurality is atleast
 2. 10. The nanostructure according to claim 1, wherein each of themetal nanoblades of the plurality is anisotropic.
 11. The nanostructureaccording to claim 1, wherein each of the metal nanoblades of theplurality exhibits a biaxial texture.
 12. The nanostructure according toclaim 1, wherein the metal nanoblades of the plurality are separatedfrom one another by between about 1 and about 2 μm.